1. Field of the Invention
The present invention relates generally to integrated circuit fabrication, and more particularly but not exclusively to techniques for preparing samples for transmission electron microscope (TEM) analysis.
2. Description of the Background Art
Fabrication of an integrated circuit device often includes analyzing topographies at different stages of the manufacturing process to insure that the device meets specified functionality requirements. By topographies it is meant a substrate or wafer, such as a semiconductor wafer, having one or more layers formed thereon, regardless of whether a functioning device has been formed from the topography or not.
A number of different techniques may be used to analyze device topography. For example, a cross-sectional analysis of the device topography may be performed using a transmission electron microscope (TEM). Generally, a TEM directs an electron beam at a prepared cross-sectional sample of the device while a photograph is simultaneously taken. Both the photograph and electron beam are used to analyze the exposed layers.
There are several different techniques or approaches for preparing cross-sectional samples of a device for TEM observation and analysis.
In mechanical polishing, the sample is sandwiched between a glass slide and silicon, and then polished using a traditional wedge polish technique.
The focused ion beam (FIB) in-situ lift-out (INLO) method takes advantage of an FIB instrument's deposition and cutting capability to attach or detach the sample to either a probe or a grid. A specialized probe in the FIB instrument's chamber is employed to move the sample around. After the sample is transferred from an original grid to a vertical grid, the FIB instrument is used to cut the sample to form a thin area for TEM observation. Systems for preparing and manipulating TEM samples by the FIB INLO method are commercially available from Omniprobe, Inc. of Dallas, Tex. Further details of the FIB INLO method are described in, for example, T. Yaguchi, Microscopy and Microanalysis, Vol. 7, No. Supp/2, p. 938, 2001, and in K. Mcllwrath and N. Wang, ISTFA 2004, p. 320.
In the Glue+LO (lift-out) method, the sample is glued to silicon with conductive glue and, after the glue has cured, the FIB instrument is used to prepare the sample using a traditional lift-out technique. Typically, the sample is lifted out using glass needle. Further details of this method are described in, for example, Jon C. Lee, Microelectronics Reliability 41 (2001) 1551-1556.
Heretofore, the above approaches have not proven wholly satisfactory for a number of reasons. In particular, sample handling in the mechanical polishing method is extremely difficult and is highly dependent on the skill of the person doing the work. The FIB INLO method requires a focused ion beam (FIB) instrument with specialized probe attachments. Moreover, constant exposure to an ion beam may cause crystal structure damage to the sample, leading to erroneous analysis. Similarly, the Glue+LO method can cause sample crystal structure damage and sample loss during the lift-out. Furthermore, the particles in the glue can cause a non-uniform sample thickness, further complicating the analysis. Accordingly, there is a need for an improved technique of preparing and manipulating samples for TEM observation and analysis.